Two-step roller finger followers (RFF) for controllably activating and deactivating compression valves in a variable valve activation train in an internal combustion engines are well known. An RFF extends between a hydraulic lash adjuster (HLA) and the stem of a valve. Engagement of the RFF with a cam lobe of an engine camshaft causes the RFF to be pivoted about the HLA and thereby to depress the valve stem, opening the valve.
A two-step RFF mechanism allows an engine valve to be operated by two different cam lobe profiles, one with first and second portions of the mechanism locked together by a slidable lock pin (typically for high lift) and the other with the mechanism portions unlocked (typically low lift).
In prior art RFFs, a known problem exists in that the lock pin may be only partially engaged with the high-lift follower portion of the RFF when a high-lift valve event begins. In some instances, there is enough engagement to begin to open the valve but not enough engagement to complete the full valve event. At some point during the valve event, the load on the lock pin becomes insupportable, ejecting the lock pin from engagement with the high-lift follower portion. The effect of this ejection event is that the valve spring compression energy is instantly released and transferred to either the lower-lift cam profile or to the valve seat.
Premature lock pin ejection is highly undesirable because a) the intended valve and engine event is frustrated, resulting in improper engine operation; b) the extreme shock produced in associated engine components may cause damage; and c) repeated ejections can damage the lock pin and the high-lift follower portion such that the RFF cannot function properly and must be replaced.
What is needed in the art is a two-step roller finger follower having a timing mechanism to ensure that a switching event can occur only immediately at the completion of a valve event, thereby maximizing the time available for the lock pin to completely translate, either into or out of locking relationship, and thus minimizing the opportunity for a lock pin ejection.
It is a principal object of the present invention to prevent lock pin ejections during operation of a two-step switchable roller finger follower in an internal combustion engine.